Tyrosine-specific protein kinases play crucial roles in transducing signals that regulate a diversity of cellular responses, including proliferation and differentiation. Protein tyrosine kinases are further implicated in neoplastic transformation by the finding that a disproportionate number of oncogenes are mutated versions of normal cellular tyrosine kinases. Cellular transformation by these oncogenes is accompanied by an increased and/or altered profile of intracellular phosphotyrosyl-proteins. These observations suggest that protein tyrosine kinases represent potential targets for a novel class of low molecular weight inhibitors directed at the signaling pathways involved in initiation and maintenance of the transformed state. We hypothesize that an effective approach to the discovery of novel anti-tumor agents will be to identify natural products with selective inhibitory effects on protein tyrosine kinase activities. To identify new protein tyrosine kinase inhibitors, we will screen extracts from higher plants using an in vitro tyrosine kinase assay. Plant extracts will be analyzed for their abilities to inhibit exogenous substrate phosphorylation by cell membranes containing the oncogenic tyrosine kinase, pp6O(v-src). Subsequently, this assay will be used to direct the isolation of active tyrosine kinase inhibitors from the crude extracts. In related studies, in vitro assays will be employed to assess the effects of limited chemical modifications of active compounds on the potency and selectivity of kinase inhibition. Active tyrosine kinase inhibitors will be further evaluated as inhibitors of protein tyrosine phosphorylation and cellular proliferation in vivo using a non-transformed, growth factor-dependent, myeloid progenitor cell line and transformed, growth factor-independent sub-lines expressing oncogenic tyrosine kinases. The ultimate goal of these studies will be to characterize novel lead compounds with selective inhibitory effects on both oncogenic tyrosine kinase activities and the un- controlled growth of cancer cells.